His harshest language came in a February 2007 speech when he said, "I think that Donald Rumsfeld will go down in history as one of the worst secretaries of defense in history."
Recently, McCain has been telling reliably conservative audiences that he believes Bush deserves credit for the fact that the United States has not been attacked by terrorists since the 9/11 catastrophe. "He sure would have gotten the blame if we had," McCain says.
Though McCain recently won the endorsement of former President George H.W. Bush, the current president has been tight-lipped about expressing favor for one potential GOP nominee over another. In an interview last month, Bush said of the eventual GOP nominee, "to the extent I can help, I want to."
As McCain emerged the obvious front-runner, Bush has stepped forward to defend the Arizona Republican's conservative credentials, stopping short of an endorsement.
Bush admitted to Fox News last month that McCain "has some convincing to do to convince people that he is a solid conservative. And I'll be glad to help him if he's the nominee because he is a conservative."
For McCain, whose weak points in the Republican base include key blocs of religious and conservative voters who cast ballots for former rival Mike Huckabee, the president's stamp of approval in the Rose Garden could go a long way.
Travel with McCain is not on Bush's schedule as of yet.
"I do expect the president will do some campaigning with Sen McCain but it's really premature for us to say," the White House press secretary said. "The President in his position needs to help all GOP candidates heading into the 08 election. "
Hitting a jocular note during the press conference, Bush he'd "find ample time to help." "I could help raising money. And if he wants my pretty face standing by his side at one of these rallies, I'll be glad to show up," Bush continued, "But they're going to be looking at him, you know. I'm going to be in Crawford with my feet up."